Various prior art means have been devised to reduce current surges associated with full-voltage starting of induction motors. Typically, a reduced stator voltage is first applied, then gradually increased in a series of manual steps or caused to rise automatically from said initially reduced voltage to full-rated voltage during a preferred time period, i.e., in the form of a "voltage increase vs. time ramp". U.S. Pat. No. 4,404,511, entitled "Motor Power Factor Controller With A Reduced Voltage Starter" is an example of a "voltage ramp-type" motor starter that also includes reduced-load power savings means that becomes operative after a manual, i.e., arbitrarily selected, start-up time period during which full power line voltage is ultimately applied to the stator windings (regardless of actual start-up requirement). As is common practice of the general prior art with reduced-voltage motor start means, said patent neither incorporates nor anticipates protection against power source anomalies or overcurrent protection of the stator winding from locked-rotor events during motor start-up (or later).
U.S. Pat. No. 4,086,406 entitled "Chilled Controls" is an example of another arbitrarily-timed "ramp-type" reduced voltage motor starter that includes means for stator overcurrent protection and AC power source anomalies, but does not incorporate means to later improve motor operating efficiency by adjusting input voltage as a function of load demand. Said motor protection functions are very complex and based upon wholly different concepts and circuit implementation means than the present invention.
Taken together, the above cited art is representative of the fundamental concepts and practices employed by the general prior art for electronic motor start control and stator overcurrent protection means. In the instance of motor start control, said prior art may be characterized as slowly increasing the magnitude of stator input voltage from a reduced level to full power line voltage as a function of time passage, wherein said time factor is arbitrarily selectable or comprises a preferred time duration predetermined by design, but, in any event, is not controllingly responsive to individual electro-mechanical characteristics of the motor actually being controlled and its actual load of a moment. In the instance of stator overcurrent protection, the prior art may be characterized as being controllingly responsive to alternation current peaks--or the average--of RMS current flow into the stator winding and/or temperature rise therein for a predetermined or manually selected time period. Prior art power source protection for power source anomalies typically rely upon a multiplicity of electronic comparators and complicated circuit arrangements.
Arbitrarily timed "stator voltage ramp-type" start control means often result in unreliable starting, motor system instability, unnecessary power waste and motor heating during excessively slow acceleration, and/or significant vibration during some portion of the motor start-up period. Likewise, they routinely permit the flow of excessive stator current and waste substantial electric power since (typically reduced) start-up motor loads seldom require the full AC power source voltage which is ultimately coupled at the termination of their reduced-voltage motor start-up time period. Effective--but nuisance free--operation of stator RMS current overprotection means that respond to alternation peaks above a particular threshold is seldom realized with electronically-reduced voltage starter means due to alternation crest factors and power source, phase-related transient stator inrush currents associated with switched-partial sine waves produced thereby.
In my prior U.S. Pat. No. 4,297,628, entitled "Energy Economizer for Induction Motors", there is disclosed AC power saving means which utilizes certain characteristics of stator inrush current related by natural motor properties to rotor mechanical load magnitude and the efficiency with which electric energy is converted into mechanical energy thereby to cause an induction motor to operate with minimum input power from zero to maximum rated load. This prior U.S. patent (4,297,628) describes stator current demodulating means for an energy saving stator power control system employing wave modifier means suitable for single phase induction motors. Said prior patent also references my U.S. Pat. No. 4,242,625 entitled "Energy Economizer for Polyphase Induction Motors" which discloses an arrangement of the same general type employing two wave modifiers which may be responsive to motor load detection means of the current demodulator-type thereby to provide load/efficiency-related control of AC power input to a three phase motor.
The object of my aforementioned prior inventions was to provide AC power control means responsive to an induction motor's natural load/efficiency-related characteristics thereby to enable said characteristics to maintain near-optimum motor efficiency from zero to maximum rated mechanical load with low, nominal or high power source voltage by varying stator input power in proportion to workload requirement. Those prior inventions do not provide motor start-up energy-saving, start-current surge reduction, protection against power source anomalies, or overcurrent protection for the stator winding(s) from a locked-rotor event during or after motor start-up. However, it is known that when full power line voltage is applied to start a motor, stator current surges from five to eight times normal operating current subject the motor to severe electromechanical stress, is wasteful of electrical power even when a motor is free to start normally, and will quickly cause permanent stator winding damage if the motor driven mechanism jams during start-up or stops rotation any time thereafter. It is also known that thermally-activated circuit breakers typically prevent catastrophic, immediate motor failure, but allow some degree of permanent winding damage which shortens a motor's useful life when a locked-rotor event or power source anomaly occurs.
The present invention comprises significant improvement in the art of induction motor starting and includes means for 3.0. motor protection from excessively high or low power source voltage, phase loss, and severe overload, including locked-rotor events, which occur during or subsequent to motor start-up. While the present invention relates generally to the field of power input controls for induction motors, it specifically relates to a motor current demodulating-type power saving controller and further provides controlled current starting by reduction of input voltage and stator winding protection from start-up phase loss and/or prolonged operation with input current substantially greater than rated full-load magnitude due to locked-rotor conditions or power source anomalies.
The present invention avoids various difficulties, limitations, energy waste, and nuisance aspects of prior art induction motor start and protection means by practical application of the fundamentally new concepts concerning stator input power control and novel circuit means for monitoring performance-related stator initial inrush current characteristics first disclosed in my U.S. Pat. No. 4,297,628. As will be described later, stator power control relies upon processing and control use of short-duration (microseconds) voltage pulses whose characteristics are related by natural motor properties to rotor speed (slip), motor load, power conversion efficiency and stator current peak amplitude during each power source alternation.